The objectives of the proposed research are to design and construct a real-time Remote Palpation imaging system, and to clinically evaluate this system in the context of breast imaging. Remote Palpation is a new imaging method that uses acoustic radiation force to characterize variations in tissue stiffness. In this method, acoustic radiation force is applied to small volumes of tissue (approximately 2 mm3), and the resulting displacement patterns are imaged using correlation based techniques. The tissue displacements are inversely proportional to the stiffness of the tissue and thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. Remote Palpation is performed using a single transducer on a modified diagnostic ultrasound scanner to both generate the high intensity focused acoustic 'pushing' beams, which create the radiation force, and to track the resulting tissue displacements. We hypothesize that Remote Palpation will provide high contrast, high resolution images of relative tissue stiffness. Possible clinical applications for Remote Palpation include: identifying and differentiating malignant lesions in the breast, liver, kidney, thyroid and prostate; and identifying and characterizing atherosclerosis. The development efforts proposed herein are focused on the breast. We hypothesize that Remote Palpation images will improve clinicians' abilities to non-invasively differentiate benign from malignant breast lesions, thus decreasing the number of biopsies performed on benign breast lesions. Remote Palpation is an entirely new imaging modality, and thus we propose to address theoretical issues (i.e. fundamental physics, signal processing, image formation, thermal modeling), practical issues (i.e. real-time implementation), and to perform clinical studies to evaluate the utility of the method. The successful completion of this project will result in a new imaging modality capable of providing information about local variations in tissue stiffness.